**3. Immunization: vaccine adjuvant-delivery system and strategies**

In designing the NP-based VADSs, what should be emphasized is that the differently sourced Ags are processed and trigger immune signal transduction in different ways [34]. Usually, the internalized exogenous Ags delivered by NPs are processed intracellularly by APCs into small antigens just inside the endolysosomal vesicles and then loaded favorably on MHC-II molecules, leading to activation of CD4+ T cells to differentiate into Th2 cells, which will further stimulate production of antibodies by B cells [8]. In contrast, the endogenous Ags, such as viral Ags, cancer components, and intracellular-degraded proteins, are usually presented in the cytosol and often loaded on MHC-I molecules, allowing for further activation of CD8<sup>+</sup> T cells to differentiate into CTLs and to engender cellular immunity [35]. However, it is generally believed that, though not well understood, provided the exogenous Ags are transported via membrane fusion or other ways engendering Ag lysosome escape into the cytosol, they can also be processed via MHC-I presentation in just the same manner as that for endogenous Ags, and this process is known as Ag cross presentation [36]. This provides the basis for designing the NP-based VADS which are exogenous particles favoring of inducing humoral immunity but may be adorned with materials that can facilitate APC internalization in the membrane fusion manner or promote endolysosome escape and cross presentation of Ags to induce cellular immunity, thus expanding VADS into various applications, including mainly prophylaxis of infections, treatment of autoimmunity diseases, and immunotherapy of cancer [12]. In particular, as a multifunctional VADS, NPs modified with different materials with intrinsic and specific adjuvanticity, such as TLRas (e.g., MPL and CpG ODN), squalene, and saponin (e.g., water-soluble QS-21), though showing individually distinctive features, share some key characteristics in immune-boosting functions [37]. Summarily, these multifunctional Nps as a VADS trigger immunoresponses with features including, mainly, early activation, though at different levels, of innate immunity, which will subsequently translate into strong antibody and cellular responses to the delivered antigens [5]; a wide breadth of adaptive immunity is able to confer protections against heterovariants of pathogens; for example, vaccines delivered by liposomes containing MPL can defend against influenza viruses or human papillomavirus (HPV) strains that are not contained in the vaccines [38]; significant enhancement of the immunoresponses and the efficacy of vaccines in the elderly who show a waning immune responsiveness to infection and vaccination, as shown for vaccines formulated with MPL-liposomes against herpes zoster virus [39]. These results, together with the feasibility of large-scale manufacture and the track records of acceptable safety profiles of many liposomebased medications, pave the way for developing novel multifunctional liposomes to be used as a VADS for producing the vaccine products fitting humans of different age against infections with a high toll of morbidity and mortality.
